Greenhouse Gases Likely Drove Near-Record U.S. Warmth in 2006

August 28, 2007

Greenhouse gases likely accounted for more than half of the widespread warmth across the continental United States last year, according to a new study by four scientists at NOAA's Earth System Research Lab in Boulder, Colo. Last year's average temperature was the second highest since record-keeping began in 1895. The team found that it was very unlikely that the 2006 El Niño played any role, though other natural factors likely contributed to the unusual warmth. The findings will appear September 5 in the Geophysical Research Letters, a publication of the American Geophysical Union.

Map showing above normal annual temperatures in 2006.
Credit: NOAA

Map showing above normal annual temperatures in 2006.

The NOAA team also found that the probability of U.S. temperatures breaking a record in 2006 had increased 15-fold compared to pre-industrial times because of greenhouse gas increases in Earth's atmosphere.

Preliminary data available last January led NOAA to place 2006 as the warmest year on record. In May, NOAA changed the 2006 ranking to second warmest after updated statistics showed the year was 0.08 degree F cooler than 1998.

The annual average temperature in 2006 was 2.1 degrees F above the 20th Century average and marked the ninth consecutive year of above-normal U.S. temperatures. Each of the contiguous 48 states reported above-normal annual temperatures and, for the majority of states, 2006 ranked among the 10 hottest years since 1895.

"We wanted to find out whether it was pure coincidence that the two warmest years on record both coincided with El Niño events," says lead author Martin Hoerling of NOAA/ESRL. "We decided to quantify the impact of El Niño and compare it to the human influence on temperatures through greenhouse gases." El Niño is a warming of the surface of the east tropical Pacific Ocean.

Using data from 10 past El Niño events observed since 1965, the authors examined the impact of El Niño on average annual U.S. surface temperatures. They found a slight cooling across the country. To overcome uncertainties inherent in the data analysis, the team also studied the El Niño influence using two atmospheric climate models. The scientists conducted two sets of 50-year simulations of U.S. climate, with and without the influence of El Niño sea-surface warming. They again found a slight cooling across the nation when El Niño was present.

To assess the role of greenhouse gases in the 2006 warmth, the NOAA team analyzed 42 simulations of Earth's climate from 18 climate models provided for the latest assessment by the Intergovernmental Panel on Climate Change. The models included greenhouse gas emissions and airborne particles in Earth's atmosphere since the late 19th century and computed their influence on average temperatures through 2006. The results of the analysis showed that greenhouse gases produced warmth over the entire United States in the model projections, much like the warming pattern that was observed last year across the country.

For a final check, the scientists compared the observed 2006 pattern of abnormal surface temperatures to the projected effects of greenhouse-gas warming and El Niño temperature responses. The U.S. temperature pattern of widespread warming was completely inconsistent with the pattern expected from El Niño, but it closely matched the expected effects of greenhouse warming.

When average annual temperature in the United States broke records in 1998, a powerful El Niño was affecting climate around the globe. Scientists widely attributed the unusual warmth in the United States to the influence of the ongoing El Niño.

"That attribution was not confirmed at the time," says Hoerling. "Now we have the capability, on the spatial scale of the United States, to better distinguish natural climate variations from climate changes caused by humans."

The authors also estimate that there is a 16 percent chance that 2007 will bring record-breaking warmth.

NOAA, an agency of the U.S. Commerce Department, is celebrating 200 years of science and service to the nation. From the establishment of the Survey of the Coast in 1807 by Thomas Jefferson to the formation of the Weather Bureau and the Commission of Fish and Fisheries in the 1870s, much of America's scientific heritage is rooted in NOAA.

NOAA is dedicated to enhancing economic security and national safety through the prediction and research of weather and climate-related events and information service delivery for transportation, and by providing environmental stewardship of our nation's coastal and marine resources. Through the emerging Global Earth Observation System of Systems (GEOSS), NOAA is working with its federal partners, more than 70 countries and the European Commission to develop a global monitoring network that is as integrated as the planet it observes, predicts and protects.

Note to Editors: Please contact Peter Weiss (pweiss@agu.org) or Anatta (anatta@noaa.gov, 303-497-6288), for a copy of the paper "Explaining the Record U.S. Warmth of 2006," M. Hoerling, et al., AGU # 2007GL030643.

More Information